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  <div class="section" id="module-hmac">
<span id="hmac-keyed-hashing-for-message-authentication"></span><h1><a class="reference internal" href="#module-hmac" title="hmac: Keyed-Hashing for Message Authentication (HMAC) implementation"><code class="xref py py-mod docutils literal notranslate"><span class="pre">hmac</span></code></a> — Keyed-Hashing for Message Authentication<a class="headerlink" href="#module-hmac" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.8/Lib/hmac.py">Lib/hmac.py</a></p>
<hr class="docutils" />
<p>This module implements the HMAC algorithm as described by <span class="target" id="index-0"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc2104.html"><strong>RFC 2104</strong></a>.</p>
<dl class="function">
<dt id="hmac.new">
<code class="sig-prename descclassname">hmac.</code><code class="sig-name descname">new</code><span class="sig-paren">(</span><em class="sig-param">key</em>, <em class="sig-param">msg=None</em>, <em class="sig-param">digestmod=''</em><span class="sig-paren">)</span><a class="headerlink" href="#hmac.new" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new hmac object.  <em>key</em> is a bytes or bytearray object giving the
secret key.  If <em>msg</em> is present, the method call <code class="docutils literal notranslate"><span class="pre">update(msg)</span></code> is made.
<em>digestmod</em> is the digest name, digest constructor or module for the HMAC
object to use.  It may be any name suitable to <a class="reference internal" href="hashlib.html#hashlib.new" title="hashlib.new"><code class="xref py py-func docutils literal notranslate"><span class="pre">hashlib.new()</span></code></a>.
Despite its argument position, it is required.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.4: </span>Parameter <em>key</em> can be a bytes or bytearray object.
Parameter <em>msg</em> can be of any type supported by <a class="reference internal" href="hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate"><span class="pre">hashlib</span></code></a>.
Parameter <em>digestmod</em> can be the name of a hash algorithm.</p>
</div>
<div class="deprecated-removed">
<p><span class="versionmodified">Deprecated since version 3.4, removed in version 3.8: </span>MD5 as implicit default digest for <em>digestmod</em> is deprecated.
The digestmod parameter is now required.  Pass it as a keyword
argument to avoid awkwardness when you do not have an initial msg.</p>
</div>
</dd></dl>

<dl class="function">
<dt id="hmac.digest">
<code class="sig-prename descclassname">hmac.</code><code class="sig-name descname">digest</code><span class="sig-paren">(</span><em class="sig-param">key</em>, <em class="sig-param">msg</em>, <em class="sig-param">digest</em><span class="sig-paren">)</span><a class="headerlink" href="#hmac.digest" title="Permalink to this definition">¶</a></dt>
<dd><p>Return digest of <em>msg</em> for given secret <em>key</em> and <em>digest</em>. The
function is equivalent to <code class="docutils literal notranslate"><span class="pre">HMAC(key,</span> <span class="pre">msg,</span> <span class="pre">digest).digest()</span></code>, but
uses an optimized C or inline implementation, which is faster for messages
that fit into memory. The parameters <em>key</em>, <em>msg</em>, and <em>digest</em> have
the same meaning as in <a class="reference internal" href="#hmac.new" title="hmac.new"><code class="xref py py-func docutils literal notranslate"><span class="pre">new()</span></code></a>.</p>
<p>CPython implementation detail, the optimized C implementation is only used
when <em>digest</em> is a string and name of a digest algorithm, which is
supported by OpenSSL.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>

<p>An HMAC object has the following methods:</p>
<dl class="method">
<dt id="hmac.HMAC.update">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">update</code><span class="sig-paren">(</span><em class="sig-param">msg</em><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.update" title="Permalink to this definition">¶</a></dt>
<dd><p>Update the hmac object with <em>msg</em>.  Repeated calls are equivalent to a
single call with the concatenation of all the arguments:
<code class="docutils literal notranslate"><span class="pre">m.update(a);</span> <span class="pre">m.update(b)</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">m.update(a</span> <span class="pre">+</span> <span class="pre">b)</span></code>.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.4: </span>Parameter <em>msg</em> can be of any type supported by <a class="reference internal" href="hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate"><span class="pre">hashlib</span></code></a>.</p>
</div>
</dd></dl>

<dl class="method">
<dt id="hmac.HMAC.digest">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">digest</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.digest" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the digest of the bytes passed to the <a class="reference internal" href="#hmac.HMAC.update" title="hmac.HMAC.update"><code class="xref py py-meth docutils literal notranslate"><span class="pre">update()</span></code></a> method so far.
This bytes object will be the same length as the <em>digest_size</em> of the digest
given to the constructor.  It may contain non-ASCII bytes, including NUL
bytes.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>When comparing the output of <a class="reference internal" href="#hmac.digest" title="hmac.digest"><code class="xref py py-meth docutils literal notranslate"><span class="pre">digest()</span></code></a> to an externally-supplied
digest during a verification routine, it is recommended to use the
<a class="reference internal" href="#hmac.compare_digest" title="hmac.compare_digest"><code class="xref py py-func docutils literal notranslate"><span class="pre">compare_digest()</span></code></a> function instead of the <code class="docutils literal notranslate"><span class="pre">==</span></code> operator
to reduce the vulnerability to timing attacks.</p>
</div>
</dd></dl>

<dl class="method">
<dt id="hmac.HMAC.hexdigest">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">hexdigest</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.hexdigest" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <a class="reference internal" href="#hmac.digest" title="hmac.digest"><code class="xref py py-meth docutils literal notranslate"><span class="pre">digest()</span></code></a> except the digest is returned as a string twice the
length containing only hexadecimal digits.  This may be used to exchange the
value safely in email or other non-binary environments.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>When comparing the output of <a class="reference internal" href="#hmac.HMAC.hexdigest" title="hmac.HMAC.hexdigest"><code class="xref py py-meth docutils literal notranslate"><span class="pre">hexdigest()</span></code></a> to an externally-supplied
digest during a verification routine, it is recommended to use the
<a class="reference internal" href="#hmac.compare_digest" title="hmac.compare_digest"><code class="xref py py-func docutils literal notranslate"><span class="pre">compare_digest()</span></code></a> function instead of the <code class="docutils literal notranslate"><span class="pre">==</span></code> operator
to reduce the vulnerability to timing attacks.</p>
</div>
</dd></dl>

<dl class="method">
<dt id="hmac.HMAC.copy">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">copy</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.copy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a copy (“clone”) of the hmac object.  This can be used to efficiently
compute the digests of strings that share a common initial substring.</p>
</dd></dl>

<p>A hash object has the following attributes:</p>
<dl class="attribute">
<dt id="hmac.HMAC.digest_size">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">digest_size</code><a class="headerlink" href="#hmac.HMAC.digest_size" title="Permalink to this definition">¶</a></dt>
<dd><p>The size of the resulting HMAC digest in bytes.</p>
</dd></dl>

<dl class="attribute">
<dt id="hmac.HMAC.block_size">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">block_size</code><a class="headerlink" href="#hmac.HMAC.block_size" title="Permalink to this definition">¶</a></dt>
<dd><p>The internal block size of the hash algorithm in bytes.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="attribute">
<dt id="hmac.HMAC.name">
<code class="sig-prename descclassname">HMAC.</code><code class="sig-name descname">name</code><a class="headerlink" href="#hmac.HMAC.name" title="Permalink to this definition">¶</a></dt>
<dd><p>The canonical name of this HMAC, always lowercase, e.g. <code class="docutils literal notranslate"><span class="pre">hmac-md5</span></code>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
</dd></dl>

<p>This module also provides the following helper function:</p>
<dl class="function">
<dt id="hmac.compare_digest">
<code class="sig-prename descclassname">hmac.</code><code class="sig-name descname">compare_digest</code><span class="sig-paren">(</span><em class="sig-param">a</em>, <em class="sig-param">b</em><span class="sig-paren">)</span><a class="headerlink" href="#hmac.compare_digest" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></code>.  This function uses an approach designed to prevent
timing analysis by avoiding content-based short circuiting behaviour,
making it appropriate for cryptography.  <em>a</em> and <em>b</em> must both be of the
same type: either <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> (ASCII only, as e.g. returned by
<a class="reference internal" href="#hmac.HMAC.hexdigest" title="hmac.HMAC.hexdigest"><code class="xref py py-meth docutils literal notranslate"><span class="pre">HMAC.hexdigest()</span></code></a>), or a <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a>.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>If <em>a</em> and <em>b</em> are of different lengths, or if an error occurs,
a timing attack could theoretically reveal information about the
types and lengths of <em>a</em> and <em>b</em>—but not their values.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.3.</span></p>
</div>
</dd></dl>

<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt>Module <a class="reference internal" href="hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate"><span class="pre">hashlib</span></code></a></dt><dd><p>The Python module providing secure hash functions.</p>
</dd>
</dl>
</div>
</div>


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